1. Field of the Invention
The present invention generally relates to ultrasonic surgical systems and, more particularly, to a method for detecting a loose blade on a hand piece connected to an ultrasonic surgical system.
2. Description of the Related Art
It is known that electric scalpels and lasers can be used as a surgical instrument to perform the dual function of simultaneously effecting the incision and hemostatis of soft tissue by cauterizing tissues and blood vessels. However, such instruments employ very high temperatures to achieve coagulation, causing vaporization and fumes as well as splattering. Additionally, the use of such instruments often results in relatively wide zones of thermal tissue damage.
Cutting and cauterizing of tissue by means of surgical blades vibrated at high speeds by ultrasonic drive mechanisms is also well known. One of the problems associated with such ultrasonic cutting instruments is uncontrolled or undamped vibrations and the heat, as well as material fatigue resulting therefrom. In an operating room environment attempts have been made to control this heating problem by the inclusion of cooling systems with heat exchangers to cool the blade. In one known system, for example, the ultrasonic cutting and tissue fragmentation system requires a cooling system augmented with a water circulating jacket and means for irrigation and aspiration of the cutting site. Another known system requires the delivery of cryogenic fluids to the cutting blade.
It is known to limit the current delivered to the transducer as a means for limiting the heat generated therein. However, this could result in insufficient power to the blade at a time when it is needed for the most effective treatment of the patient. U.S. Pat. No. 5,026,387 to Thomas, which is assigned to the assignee of the present application and is incorporated herein by reference, discloses a system for controlling the heat in an ultrasonic surgical cutting and hemostasis system without the use of a coolant, by controlling the drive energy supplied to the blade. In the system according to this patent an ultrasonic generator is provided which produces an electrical signal of a particular voltage, current and frequency, e.g. 55,500 cycles per second. The generator is connected by a cable to a hand piece which contains piezoceramic elements forming an ultrasonic transducer. In response to a switch on the hand piece or a foot switch connected to the generator by another cable, the generator signal is applied to the transducer, which causes a longitudinal vibration of its elements. A structure connects the transducer to a surgical blade, which is thus vibrated at ultrasonic frequencies when the generator signal is applied to the transducer. The structure is designed to resonate at the selected frequency, thus amplifying the motion initiated by the transducer.
The signal provided to the transducer is controlled so as to provide power on demand to the transducer in response to the continuous or periodic sensing of the loading condition (tissue contact or withdrawal) of the blade. As a result, the device goes from a low power, idle state to a selectable high power, cutting state automatically depending on whether the scalpel is or is not in contact with tissue. A third, high power coagulation mode is manually selectable with automatic return to an idle power level when the blade is not in contact with tissue. Since the ultrasonic power is not continuously supplied to the blade, it generates less ambient heat, but imparts sufficient energy to the tissue for incisions and cauterization when necessary.
The control system in the Thomas patent is of the analog type. A phase lock loop (that includes a voltage controlled oscillator, a frequency divider, a power switch, a matching network and a phase detector), stabilizes the frequency applied to the hand piece. A microprocessor controls the amount of power by sampling the frequency, current and voltage applied to the hand piece, because these parameters change with load on the blade.
The power versus load curve in a generator in a typical ultrasonic surgical system, such as that described in the Thomas patent, has two segments. The first segment has a positive slope of increasing power as the load increases, which indicates constant current delivery. The second segment has a negative slope of decreasing power as the load increases, which indicates a constant or saturated output voltage. The regulated current for the first segment is fixed by the design of the electronic components and the second segment voltage is limited by the maximum output voltage of the design. This arrangement is inflexible since the power versus load characteristics of the output of such a system can not be optimized to various types of hand piece transducers and ultrasonic blades. The performance of traditional analog ultrasonic power systems for surgical instruments is affected by the component tolerances and their variability in the generator electronics due to changes in operating temperature. In particular, temperature changes can cause wide variations in key system parameters such as frequency lock range, drive signal level, and other system performance measures.
In order to operate an ultrasonic surgical system in an efficient manner, during startup the frequency of the signal supplied to the hand piece transducer is swept over a range to locate the resonance frequency. Once it is found, the generator phase lock loop locks on to the resonance frequency, continues to monitor the transducer current to voltage phase angle, and maintains the transducer resonating by driving it at the resonance frequency. A key function of such systems is to maintain the transducer resonating across load and temperature changes that vary the resonance frequency. However, these traditional ultrasonic drive systems have little to no flexibility with regards to adaptive frequency control. Such flexibility is key to the system""s ability to discriminate undesired resonances. In particular, these systems can only search for resonance in one direction, i.e., with increasing or decreasing frequencies and their search pattern is fixed. The system cannot: (i) hop over other resonance modes or make any heuristic decisions, such as what resonance to skip or lock onto, and (ii) ensure delivery of power only when appropriate frequency lock is achieved.
The prior art ultrasonic generator systems also have little flexibility with regard to amplitude control, which would allow the system to employ adaptive control algorithms and decision making. For example, these fixed systems lack the ability to make heuristic decisions with regards to the output drive, e.g., current or frequency, based on the load on the blade and/or the current to voltage phase angle. It also limits the system""s ability to set optimal transducer drive signal levels for consistent efficient performance, which would increase the useful life of the transducer and ensure safe operating conditions for the blade. Further, the lack of control over amplitude and frequency control reduces the system""s ability to perform diagnostic tests on the transducer/blade system and to support troubleshooting in general.
Some limited diagnostic tests performed in the past involve sending a signal to the transducer to cause the blade to move and the system to be brought into resonance or some other vibration mode. The response of the blade is then determined by measuring the electrical signal supplied to the transducer when the system is in one of these modes. The ultrasonic system described in U.S. application Ser. No. 09/693,621, filed on Oct. 20, 2000, which is incorporated herein by reference, possesses the ability to sweep the output drive frequency, monitor the frequency response of the ultrasonic transducer and blade, extract parameters from this response, and use these parameters for system diagnostics. This frequency sweep and response measurement mode is achieved via a digital code such that the output drive frequency can be stepped with high resolution, accuracy, and repeatability not existent in prior art ultrasonic systems.
A problem associated with the prior art ultrasonic systems is blade breakage or cracking at points of high stress on the blade. Breakage and cracking of blades are two major causes of the ultrasonic generator failing to acquire lock or failing to maintain longitudinal displacement. For example, as the crack develops both the frequency of oscillation and the magnitude of mechanical impedance change to such an extent that the ultrasonic generator can no longer locate the resonance of the hand piece/blade. A more advanced generator may be able to lock onto a transducer coupled to such a blade. However, a cracked blade has a reduced ability to oscillate in the longitudinal direction. In this situation, an increased ability to locate the desired resonance upon which to lock is not useful, and may actually mask the loss of optimal cutting conditions.
Further, gunked blades, i.e., blades with dried blood, skin, hair and desiccated tissue built up around the blade, present a greater load than clean blades. In particular, the gunk results in a load on the blade, and represents an increase in the mechanical impedance of the transducer presented to the ultrasonic generator.
This phenomenon has the following unwanted consequence. Ultrasonic generators possess a maximum operating voltage beyond which optimal operation of the hand piece/blade is lost. Many ultrasonic drivers attempt to maintain a constant drive current level to the transducer to keep the displacement at the blade tip constant in the presence of varying loads on the blade. As the impedance of the transducer is increased (as a result of tissue pressure, gunked tissue, etc.), the drive voltage must be increased to maintain the drive current at a constant level. Eventually, the loading of the blade becomes great enough such that the voltage reaches a maximum level, and any further loading of the blade results in a reduction of the drive current signal level.
As the current level of the drive signal is reduced, the displacement will begin to fall. The generator can drive an increasing load only as long as the hand piece/blade is not loaded such that the resonance point becomes unrecognizable (due to degradation of the signal to noise ratio or an inability of the hand piece/blade to resonate). As a consequence, the tissue applied force at maximum power, the maximum tissue applied force before losing the resonance signal, and the cutting/coagulating ability of the blade between these two operating points, become degraded.
In addition to the problems associated with loads on the blade, there is a buildup of heat at the coagulum. This buildup absorbs energy from the blade, and heats both the blade and sheath at this location. A cracked or broken blade loses the ability to resonate as well as a blade which is in good condition, and thus should be discarded. However, a gunked blade can be cleaned or used, and resonates as well as a new blade. In an operating room, access to either cracked and gunked blades for visual inspection is not practical. However, it is advantageous to differentiate between broken blades and those which are gunked, but otherwise in good condition, because a user can quickly and with confidence decide whether to discard or to clean an expensive blade. Cleaning a blade which is gunked verses discarding what is otherwise a good blade results in a substantial reduction in purchasing costs which are passed on to hospital patients as a savings.
Impedance measurements of mechanical or acoustic systems obtained at high excitation levels provides much more information than impedance measurements obtained at low excitation levels. Moreover, comparisons of impedance measurements between low and high excitation levels provide even more detailed information about the condition of the hand piece/blade. The condition of the hand piece/blade falls into three main categories.
Firstly, gunked blades and new clean blades belong to the same category because silicon anti-node supporters and other mechanical inefficiencies, such as mechanical resistance in the longitudinal direction of the blade, have the same dampening effect as gunk upon the hand piece/blade. In particular, clean/gunked systems become much better resonators as the excitation amplitude is increased, that is they become higher Q systems (the minimum impedance gets markedly lower and the maximum phases get markedly higher; see FIG. 1 and compare the impedance vs. frequency plot shown in B to the impedance vs. frequency plot shown in E, and see FIG. 2 and compare the phase vs. frequency plot shown in H to the phase vs. frequency plot shown in K). The degree of improvement is relative to the loading effect of the gunk involved. As the excitation level changes, there is a minimal change in the resonance frequency which is close to the resonance frequency of a clean hand piece/blade. At a low excitation level, such as 5mA, a cracking or slightly cracked blade is generally self healing and looks very much like a gunked blade (see FIG. 1 and compare the impedance vs. frequency plot shown in A to the impedance vs. frequency plot shown in B, and see FIG. 2 and compare the phase vs. frequency plot shown in G to the phase vs. frequency plot shown in H). The self healing characteristic, in which at a molecular level the blade becomes more homogeneous if not overly excited, results in an optimally tuned system. At low excitation levels, the surfaces at the interface of the crack do not behave like disjoint surfaces, and are held in close contact to each other by the parts of the blades which are still intact. In this situation, the system appears xe2x80x9chealthy.xe2x80x9d
Secondly, at larger excitation levels, such as 25 mA or greater, stresses at the crack become large enough such that the portion of the blade which is distal to the crack no longer acts as if it is intimately connected to the proximal portion of the blade. A characteristic of these hand piece/blades is their non-linear behavior (i.e., very sharp non-continuous changes in impedance magnitudes and phase) which occur as the resonance frequency is approached and the stresses in the shaft of the hand piece become large. As the frequency approaches resonance of the xe2x80x9cintact bladexe2x80x9d, the stresses become increasingly greater until at a certain point the blade suddenly becomes disjointed at the crack. This effectively shortens the blade, and the resonator or blade will possess completely different resonance impedance characteristics. Typically, the impedance of such a shorter blade results in a hand piece/blade which possesses a lower Q, as well as a lower frequency of resonance (see FIG. 1 and compare the respective impedance vs. frequency plots shown in A and C to the respective impedance vs. frequency plots shown in D and F, and see FIG. 2 and compare the respective phase vs. frequency plots shown in G and I to the respective phase vs. frequency plots shown in J and L).
Lastly, severely cracked blades include, but are not limited to, blades having tips which have completely fallen off due to mechanical stress acting on the blades. These blades are substantially equivalent to gunked blades. However, they are not useful for cutting/coagulating tissue in longitudinal directions. Such blades appear to behave similarly in that they present improved (if only marginally) impedance characteristics at higher excitation levels, and their frequency of resonance is not affected by higher excitation levels. However, they can be differentiated from gunked blades due to their extremely high impedance level. This requires absolute measurements, but only coarse levels of precision are required. Generally, the resonance frequency of the transducer or blade is shifted far away from the normal resonance that is typically used for a specific ultrasonic system. This shift is usually a downward shift of the resonance frequency of about 2 kilohertz. When excited with a higher level of current and compared with a lower level of current, the impedance magnitude, resonance frequency and maximum phase at resonance are quantitatively far different than the corresponding characteristics of blades which are only gunked (see FIG. 3 and compare the impedance vs. frequency plot shown in M to the impedance vs. frequency plot shown in N, and compare the phase vs. frequency plot shown in O to the phase vs. frequency plot shown in P). In this case, the hand piece/blade typically possesses a magnitude of impedance at resonance which is approximately 400 ohms higher for cracked blades than that of heavily gunked but otherwise good blades.
Most broken or cracked blades have selfhealing characteristics associated with them. The self healing characteristic, in which at a molecular level the blade becomes more homogeneous if not overly excited, results in an optimally tuned system. This homogeneity is disturbed at a high excitation level, resulting in an untuned system. When cracked or broken blades are un-energized for an extended period of time, or if energized at a low intensity for a period of time, such blades present a mechanical impedance to the ultrasonic generator that is closer to the mechanical impedance which is exhibited by an unbroken blade. At high excitation levels, the portion of the blade distal to the crack is no longer intimately connected to the hand piece/blade. The effect of the high excitation level upon the blade is that the portion of the blade proximal to the crack xe2x80x9cbangsxe2x80x9d against the portion of the blade distal to the crack, which causes a loading effect which is greater than the loading effect at low excitation displacement levels.
In other words, in the frequency range of approximately 1,000 Hz, centered around the resonance frequency of an unbroken blade, the same type of broken blade will exhibit one impedance sweep characteristic at a low voltage excitation of the drive transducer and another at a high voltage excitation level. In contrast, an unbroken blade exhibits the same impedance at both excitation levels, as long as the impedance measurement is performed quickly enough, or at a low enough displacement level such that the transducer or the blade does not overheat. Heat causes the resonance point to shift downwards in frequency. This heating effect is most prevalent when the magnitude of the excitation frequency approaches the resonance frequency due to gunk.
In addition, an excitation threshold exists, below which the blade xe2x80x9cself healsxe2x80x9d and presents increasinglyxe2x80x9ctunedxe2x80x9d impedance levels (over time) to the driving elements, and above which the crack presents a discontinuity to the homogeneity of the blade. Thus, below this threshold, the impedance characteristic may exhibit the same characteristic for all excitation levels. The blade may also appear to be healing itself at these lowered excitation levels. Above this excitation threshold, the impedance may possess a different appearance than the low impedance measurements, but may still not change with increasing levels of excitation. This excitation threshold is different for each type of blade as well as each cracked location on the blade, and is modulated by the amount of gunk loading the distal part of the blade.
Some of the impedance differences seen in a system containing a broken blade (which are not seen in a system containing an unbroken blade), when first driven with a low excitation current and then with a high excitation current, are a lower Q (i.e., a lower minimum impedance) over a frequency span centered about the resonance frequency of an unbroken blade, i.e., a higher minimum impedance and/or a lower maximum impedance. It could also mean a higher xe2x80x9cphase marginxe2x80x9d, i.e., Fa-Fr (where Fa-Fr is anti-resonance frequency minus the resonance frequency, respectively). Other differences are a higher impedance at a frequency slightly above the anti-resonance frequency of the normally operating system, a higher impedance at a frequency slightly below the resonance point of a properly working system, or a large change in the resonance frequency. Gunked or loaded blades connected to a drive system exhibit somewhat opposite effects to that of a cracked blade. A system loaded in this manner exhibits an increasingly improved Q around the resonance point as the excitation voltage is increased.
Previous attempts to differentiate between gunked and cracked blades have been based on one of two theories. First, there is a set of resonant frequencies and magnitudes of impedances which can be tabulated and used to uniquely classify each type of situation in question (broken verses gunk blades, etc.). Second, there is an impedance signature characteristic of each family of blades which can be programmed into the generator for recognition purposes. These approaches, however, require large computations of data. Complicating the factors which need to be considered are: the many types of blades presently in use and the future blades which may be developed, the quickly changing temperature of the blades and/or the hand pieces during use, the age of the piezo-electric material, the self healing affect over time of slightly cracked blades and the requirement for an absolute impedance measurement which requires expensive and complicated measurement equipment which must be kept calibrated. Another complicating factor is the simulation and recordation of the impedance characteristics of all the different types of cracks on each blade which simulate the effects of gunk. These complications make it impossible, or at least impractical, to implement a tabulation/recognition methodology for use with present ultrasonic systems.
Detection of debris on the blade, and the determination of the condition of tissue that the blade is in contact with are additional problems associated with conventional ultrasonic systems. Some ultrasonic blades are equipped with a sheath which covers the blade. The majority of the sheath is not in contact with the blade. Space (voids) between the sheath and the blade permits the blade to move freely. During use, this space can become filled with debris such as blood and tissue. This debris has a tendency to fill the space between the sheath and blade, and increase mechanical coupling between the blade and the sheath. As a result, undesired loading of the blade may increase, the temperature of the blade sheath may increase and the energy delivered to the tip may be reduced. In addition, if the debris sufficiently coagulates/hardens inside the sheath, the ability of the generator to initiate blade vibration while in contact with tissue may be prohibited. Moreover, vibration/start up of the blade in free air may also be inhibited. It is also desirable to know the relative condition of skin tissue, especially the condition of the tissue which has been altered by ultrasonic energy. Assessing the condition of tissue permits the proper adjustment of the energy applied to the tissue, and also permits the indication of when adequate cauterization, dessication, or other tissue effects have occurred. Together, these provide a means to determine whether additional energy or whether an extension of the application time of the energy is required. Further, the assessment of the tissue condition permits the avoidance of insufficient energy applications and insufficient tissue effects (i.e., poor tissue coagulation or poor tissue cauterization), which prevent application of excessive amounts of ultrasonic energy to the skin tissue which can harm surrounding tissue in the area of blade usage.
There are other problems associated with the conventional ultrasonic surgical systems. For instance, an insufficient application of torque to the blade when mounting it to a hand piece can cause squealing and prevent the ultrasonic generator from acquiring lock, prevent transmission of energy to the area of the patient being treated, or lead to a large amount of energy dissipation at the loose interface of the hand piece and blade, which cause a degradation of the overall performance of the system and the generation of unwanted heat. Similar symptoms are also associated with broken blades and/or gunked blades, i.e., blades that have become clogged or loaded with debris, such as blood and tissue.
When a high drive current is used to excite the blade, a loose connection will produce a xe2x80x9cjitteryxe2x80x9d impedance response as shown in FIG. 4, where fr1 is a resonance frequency associated with a first frequency sweep, fr2 is a resonance frequency associated with a second frequency sweep and fr3 is a resonance frequency associated with a third frequency sweep. This occurs as a result of the change in mechanical integrity of an attachment, such as a blade, which in turn causes the blade to become more tightly or less tightly connected to the hand piece. As a result, each time the drive signal is swept through the applicable frequency range, a different frequency of resonance is measured, and the magnitude and phase of impedance measurements appears xe2x80x9cnoisyxe2x80x9d.
The squealing, heating and loss of tissue effects associated with a loose blade is repaired by tightening the blade. Accordingly, it is desirable to differentiate the occurrence of faults associated with a loose blade from other more serious faults (e.g., a broken blade or debris loaded blade). It is therefore apparent that there is a need for a method for identifying the occurrence of a loose blade in a hand piece to permit rapid repairs to the ultrasonic system.
The present invention is a method for detecting loose blades in a hand piece connected to an ultrasonic surgical system. Generally, impedance measurements which are obtained at high excitation levels provide a greater amount of information than impedance measurements which are obtained at low excitation levels. In accordance with the invention, to detect a loose connection, a number of frequency sweeps of a drive signal is performed through a range which includes the resonance of a hand piece/blade system and the resonance of the hand piece without a blade attached. The frequencies of resonance in each sweep are compared. A substantial difference between the frequency sweeps is indicative of a loose blade, and a xe2x80x9cTighten Bladexe2x80x9d message is displayed on a liquid crystal display on the ultrasonic generator console.
In an alternative embodiment of the invention, the RMS noise in the magnitude of the impedance spectrum and/or phase in the impedance spectrum is measured. A large noise value is an indication that the blade is improperly attached to the hand piece.
The invention permits rapid and easy diagnosis of loose blade connections. The method of the invention also assists a surgeon or nurse in knowing when to tighten the blade, as opposed to performing more lengthy and detailed diagnostic or cleaning procedures. In addition, the time and costs associated with the diagnostic procedures to isolate faults are eliminated because, upon determining that the blade is not loose, the surgeon and nurse can request a new blade based on the assumption that the blade is broken.